SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation (Details) customer in Millions	3 Months Ended
Mar. 31, 2017 customer utility
Schedule of Equity Method Investments [Line Items]
Number of electric and natural gas customers \| customer	3.7
Connecticut, Massachusetts and New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Number of regulated utilities \| utility	6

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration